An electrophotographic technology has found widespread applications in the field of not only copying machines but also various printers and printing machines in recent years because it can provide an image of immediacy and high quality.
As for the photoreceptor which is the core of the electrophotographic technology, use of photoreceptors using organic photoconductive materials having advantages of retaining no pollution, ensuring easy film-forming, being easy to manufacture, and the like, has been the main stream in recent years instead of conventional inorganic photoconductive materials such as selenium, an arsenic-selenium alloy, cadmium sulfide or zinc oxide.
As the layer structure of the organic photoreceptor, there are known a so-called monolayer type photoreceptor obtained by dispersing a charge generation material in a binder resin, and a lamination type photoreceptor obtained by laminating a charge generation layer and a charge transport layer. The lamination type photoreceptor has been widely used because a stable high sensitivity photoreceptor can be provided by combining optimum layers of a charge generation material and a charge transport material each having a high efficiency, and characteristics are easily adjusted because of its wide material selection range. The monolayer type photoreceptor is slightly inferior to the lamination type photoreceptor in view of electric characteristics and its narrow material selection range, and accordingly has been used to a limited extent.
Further, the electrophotographic photoreceptor is repeatedly used in an electrophotographic process, i.e., in cycles of charging, exposure, development, transfer, cleaning, charge removal, and the like, during which it is subjected to various stress and will be deteriorated. Among such deteriorations, chemical deterioration may be a damage to a photosensitive layer by strongly oxidizing ozone or NOx risen from, for example, a corona charger commonly used as a charger. Thus, when the photoreceptor is repeatedly used, deterioration of electrical stability such as a reduction in the triboelectricity and an increase in the residual potential and accompanying image failure may occur. These are greatly due to chemical deterioration of a charge transport material contained in a large amount in the photosensitive layer.
Further, high sensitivity and high-speed response are required along with speeding up of the electrophotographic process in recent years. Among them, for high sensitivity, not only optimization of the charge generation material but also development of a charge transport material which well matches the charge generation material, has been required, and for high-speed response, development of a charge transport material having high mobility and showing a sufficiently low residual potential at the time of exposure has been required. In many cases, high sensitivity and high-speed response can be attained by increasing the content of a charge transport material against the binder resin. However, a photosensitive layer wherein the content of a charge transport material is large against a binder resin, has a problem such that the mechanical durability of the photosensitive layer tends to be poor in many cases, and so-called print durability for forming images repeatedly tends to deteriorate. Accordingly, a charge transport material is desired which makes high sensitivity and high-speed response possible even with an electrophotographic photoreceptor having a small content of the charge transport material in its photosensitive layer.
With a photoreceptor having a photosensitive layer having a small content of a charge transport material, a problem of leaking has been overcome, but it has been pointed out that the characteristics of the electrophotographic photoreceptor are likely to be substantially changed due to a change in the environment (the temperature, humidity, etc.), thus leading to an image defect (e.g. Patent Document 1). Further, with conventional charge transport materials, it is known that they tend to be deteriorated when exposed to an oxidizing gas represented by ozone, NOx or the like, and the durability was poor in repeated use especially when the environment in which the electrophotographic photoreceptor was used, was changed.
Patent Document 1: JP-A-2001-056595